38 BULLETIN" 314, U. S. DEPARTMENT OP AGRICULTURE. 



porcelain evaporating dish and evaporated on a steam bath. The 

 most scrupulous care must be taken at all times that no flames are 

 in its immediate vicinity, Evaporation is carried on at a gentle 

 heat, with continual stirring, until foaming practically ceases. It is 

 advisable to have a large watch glass at hand to smother the names 

 quickly should the material ignite. As the foaming subsides, the 

 heat of the steam bath may be gradually raised, and evaporation is 

 continued until the bubbles beaten or stirred to the surface of the 

 bitumen fail to give a blue flame or odor of sulphur dioxide when 

 ignited by a small gas jet. The dish of bitumen should then be set 

 in a hot-air oven maintained at 105° C. for about an hour, after which 

 it is allowed to cool. Its general character is noted and any tests 

 for bitumens that are necessary are then made upon it. 



GRADING THE MINERAL AGGREGATE. ' 



EQUIPMENT. 



1 set of 8-inch stone sieves with circular openings of 1J, if, 1, f, £, and \ inches, 



respectively. 

 1 set of 8-inch brass sand sieves of 10, 20, 30, 40, 50, 80, 100, and 200 mesh, respectively, 



with pan and cover. 

 1 rough balance, capacity 1 kilogram, sensitive to 0.1 gram. 

 1 lj-inch stiff flat brush. 

 Several sheets of nmnila paper. 



METHOD. 



While a mechanical sifter is employed in the Office of Public 

 Roads and Rural Engineering, the following hand method is given 

 for the benefit of those who have not a machine of this character 

 available. When a machine is used its method of operation should 

 be checked against the hand method described below to obtain 

 practically equivalent results, 



For aggregates containing particles too large to pass a 10-mesh 

 screen, the stone sieves are used, and are stacked in their regular order 

 over a sheet of heavy paper, with the largest size required on top. 

 The weighed amount of stone is placed on the largest sieve and is care- 

 fully protected from drafts which might carry away any of the fine 

 material. The upper sieve is then removed from the stack and shaken 

 over a large sheet of paper until no more particles come through. 

 The material thus retained, including any fragments caught in the 

 meshes of the sieve, is weighed and that which passes is added to the 

 contents of the succeeding sieve. This operation is repeated with 

 each succeeding sieve. 



When grading sands or fine aggregates, it is customary to take a 

 100-gram sample in order that the weights may give direct percentages 

 to tenths of 1 per cent. The sieves are stacked in regular order with 

 the 200-mesh sieve resting on the pan. The sample is brushed on the 



